Initial scaffold: Sinatra map app with Tauri desktop shell

Sinatra backend serving a basic Leaflet map, wrapped in a Tauri
desktop shell that starts the same server in a native window.

config/puma.rb

@@ -0,0 +1,6 @@
+bind "tcp://127.0.0.1:#{ENV.fetch('PORT', 4567)}"
+
+environment ENV.fetch("RACK_ENV", "development")
+
+workers 0
+threads 1, 4